This invention relates to a GPS (Global Positioning System) receiver which performs positioning by utilizing electromagnetic waves transmitted from NAVSTAR satellites constituting a global positioning satellite system.
Recently, GPS receivers have been highlighted in the field of navigation. Known as the GPS receiver are a multi-channel type receiver disclosed in, for example, "GDM/GPS Receiver Hardware Implementation", NAECON, 1977, pp 303-309 and a multiplex type receiver disclosed in, for example, "Applications of a Multiplexed GPS User Set", GLOBAL POSITIONING SYSTEM, Vol. II, 1984, pp 61-77. In these types of GPS receiver, a difference between a satellite signal and a signal generated in the receiver is constantly observed for four individual satellites and therefore the time at which each of the four satellites actually transmits an electromagnetic wave can be measured with high accuracies. With this construction, however, four demodulators (i.e, four mixers and three lowpass filters) are needed for each channel and as a result, the size of a receiver circuit is increased, leading to an expensive receiver of large size and large power consumption.
To deal with this problem, a receiver of small circuit size using a single demodulator for each channel is conceivable. But the single demodulator is switched to individual satellites at a timing predtermined for each satellite and a satellite signal from each satellite can not be acquired at a desired timing, with the result that it is very difficult to accurately acquire a timing for transmission of data from a satellite. At an instant, only one satellite is permitted to be observed and it takes a long time (such as of about four times of that in the multi-channel system) to track a satellite signal. Further, two numerical control oscillators are always required.